1. Field of the Invention
The present invention relates to solder alloy compositions, more particularly to a low melting point, lead-free solder alloy composition provided with improved properties, primarily in mechanical strength.
2. Description of the Related Art
When electronic parts, etc., are electrically connected to and are fixed on circuit boards and the like, a Snxe2x80x94Pb eutectic solder is most widely used at present. The Snxe2x80x94Pb solder has a eutectic alloy composition of 63 percent by weight of Sn and 37 percent by weight of Pb, and has a eutectic temperature of 183xc2x0 C. The Snxe2x80x94Pb eutectic solder alloy composition exhibits superior performance in many respects, such as temperature characteristics, soldering characteristics, and reliability, and forms the basis of mounting technology in the electronics industry.
Recently, wastes containing Pb have become a problem from the point of view of environmental protection and the enactment of lead-control regulations is anticipated in the United States, Europe, and Japan. The Snxe2x80x94Pb eutectic solder will also be regulated.
Accordingly, a lead-free solder is desired, which has equivalent performance to that of the Snxe2x80x94Pb eutectic solder in workability, such as soldering characteristics, temperature characteristics, and reliability.
Alloy compositions proposed up to the present as candidates for a lead-free solder are mainly composed of Sn, such as low melting point alloys including a Snxe2x80x94Bi alloy (57 percent by weight of Bi and a eutectic temperature of 139xc2x0 C.) and a Snxe2x80x94In alloy (52 percent by weight of In and a eutectic temperature of 117xc2x0 C.). In addition, there is a Snxe2x80x94Ag alloy with a relatively high melting point (3.5 percent by weight of Ag and a eutectic temperature of 221xc2x0 C.).
When compared with the Snxe2x80x94Pb eutectic solder, however, all compositions described above have problems, such as decrease of wettability, decrease of fatigue strength, generation of dross (an oxide film on surfaces of molten solder), and higher cost. Among the compositions described above, the Snxe2x80x94Ag alloy, even though it has a drawback of having a relatively higher melting point, is regarded as the most likely candidate by virtue of superior properties of oxidation resistance and soldering characteristics. A phase diagram of the Snxe2x80x94Ag alloy is shown in the FIGURE.
A Snxe2x80x94Ag alloy having Bi and Cu, 90Sn-7.5Bi-2Ag-0.5Cu (each numeral represents percent by weight), is named Alloy-H and the melting point thereof decreases to approximately 210xc2x0 C. While having good thermal cycle characteristics, Alloy-H is hard and brittle due to a large amount of Bi contained, so that it is difficult to make a solder wire. A problem of lift-off is also noted.
Consequently, the inventors of the present invention proposed a lead-free Snxe2x80x94Ag alloy solder disclosed in Japanese Patent Application No. 9-348212, in which the Bi content was reduced to approximately half, that is, 4 percent by weight, and a small amount of Ge was added to compensate for the reduced workability and reliability. A typical alloy composition is a five-component alloy of 93. 4Sn-2Ag-4Bi- 0.5Cu-0.1Ge (each numeral represents percent by weight).
The five-component lead-free alloy solder shows superior properties; however, degradation of properties is observed when used in a flow process and an elongation thereof is relatively small. Moreover, application of the alloy solder was limited by the extremely high cost of Ge.
Accordingly, taking the problems in the conventional technologies into consideration, it is an object of the present invention to provide a lead-free solder composition having superior mechanical properties including stress and elongation, and having properties equivalent to those of conventional lead-free solder in a melting point and wettability.
The inventors accomplished the present invention through discovering that good results may be obtained by adding a rare earth element to a Snxe2x80x94Ag alloy or a Snxe2x80x94Agxe2x80x94Bixe2x80x94Cu alloy in the course of evaluating properties thereof by adding various chemical elements.
To this end, in one aspect of the present invention, there is provided a solder alloy composition comprising a Snxe2x80x94Ag alloy and a rare earth element.
The composition of the Snxe2x80x94Ag alloy as a base alloy is preferably approximately 0.1 to 6.0 percent by weight of Ag-and the balance is substantially Sn, that is, the Snxe2x80x94Ag alloy composition is preferably significantly similar to a eutectic composition. When the content of Ag does not reach the value mentioned above, wettability will decrease and melting point will increase.
In contrast, when the content of Ag exceeds the value mentioned above, needle crystals will precipitate and mechanical strength will decrease. In addition, melting point will increase.
In another aspect of the present invention, there is provided another solder alloy composition comprising a Snxe2x80x94Agxe2x80x94Bixe2x80x94Cu alloy and a rare earth element.
The composition of the Snxe2x80x94Agxe2x80x94Bixe2x80x94Cu alloy as a base alloy is preferably not more than 6.0 percent by weight of Ag, not more than 8.0 percent by weight of Bi, and not more than 5.0 percent by weight of Cu, and the balance is substantially Sn.
The composition of the Snxe2x80x94Agxe2x80x94Bixe2x80x94Cu alloy is more preferably 1.5 to 3.0 percent by weight of Ag, 0.5 to 5.0 percent by weight of Bi, and 0.1 to 3.0 percent by weight of Cu, and the balance is substantially Sn.
When the content of Ag does not reach the value mentioned above, an angle of contact becomes larger in wettability evaluation and good wettability will not be obtained. In addition, melting point will increase.
Meanwhile, when the content of Ag exceeds the value mentioned above, the melting point will also increase, needle crystals will be observed, and mechanical strength will decrease.
When the content of Bi does not reach the value mentioned above, melting point will increase and good wettability will not be obtained.
When the content of Bi exceeds the value mentioned above, the solder alloy composition is brittle and a disconnection thereof may occur by mechanical impact or the like.
In contrast, when the content of Cu does not reach the value mentioned above, melting point will not decrease and good wettability will not be obtained.
When the content of Cu exceeds the value mentioned above, a xcex7-type intermetallic compound of Cuxe2x80x94Sn will form and will precipitate, so that cracks will easily occur.
Moreover, melting point will increase and wettability will decrease.
In each solder alloy composition, the phrase xe2x80x9cthe balance being substantially Snxe2x80x9d means that the balance is substantially pure Sn, which may contain incidental impurities.
An amount of a rare earth element is preferably not more than 5 percent by weight of each of solder alloy compositions, Snxe2x80x94Ag alloy and Snxe2x80x94Agxe2x80x94Bixe2x80x94Cu alloy.
Even a small amount of approximately 0.1 percent by weight of a rare earth element contributes to improved mechanical strength. When the amount of a rare earth element exceeds 5 percent by weight; however, effects of decreased oxidation resistance, wettability, and mechanical strength begin to appear.
In each solder alloy composition, a rare earth element to be added is preferably selected from the group consisting of La, Ce, Pr, Nd, Sm, and Gd. One or more rare earth elements are to be added to each solder alloy composition.
The mechanism of the effects caused by adding a rare earth element to the solder alloy composition according to the present invention is not clearly understood; however, addition of a proper amount of a rare earth element is capable of improving mechanical strength while maintaining nearly the same melting point and wettability.